In the traditional powder-metallurgy art, it has long been a practice to heat a precompacted body of particulate material to obtain a sintered object therefrom. According to the known process, however, sintered end products of desired quality may not be obtained and certain materials may not lend themselves to satisfactory sintering. In another known sintering technique which is commonly called "hot pressing", a mass of particulate material is placed in a mold such as of graphite and therein compressed between a pair of plungers while simultaneously being heated with heating current passing directly through the mass, through the mold or through an induction heating coil arranged to surround the mass and mold. The hot pressing is sometimes carried out isostatically or semi-isostatially with compression force applied uniformly to the mass peripherally toward central point or axis thereof.
In any of the known sintering techniques, however, a problem has now been recognized that directional variations develop in the quality of a sintered product due to the fact that sintering crystals in the mass are forced to grow with their easy-to-slip surfaces aligned in the direction in which the pressure is applied to the mass. Thus, insufficiencies in the density and lack of uniformity in the quality of sintered products result, this being particularly noticeable where the sintering particles have tendency to generate a large quantity of gaseous decompositions.